Historically, electroceramic actuators such as piezoelectric lead-zironcate titanate (PZT) or electrostrictive lead-magnesium-niobate (PMN) were operated in a configuration such that the electrodes were connected electrically in parallel and the ceramic layers were connect mechanically in series. See U.S. Pat. No. 4,932,119. The longitudinal strain component was utilized to produce the actuator displacement and force. For electric fields of 30V/mil (1×106 V/mm), less than 0.10 —m per layer was attained for a 0.007 inch active layer thickness (consistent with a strain of 800 ppm). To attain free actuator strokes approaching 10-m nearly 100 active layers were required to produce the necessary electrostrain. The net result is a complex mutilayer actuator configuration which is very sensitive to electrostrain induced stress failure. Electrically induced strain and stress in ceramic transducers are principle components in the fatigue, degradation, and eventual breakdown of electroceramic actuators. The basic failure mechanism involves the transverse component of electrostrain. Flaws in the ceramic structure such as voids and impurities also create localized regions of very large stresses. PMN being electrostrictive by its very nature has a longitudinal strain component parallel to the applied electric field direction (or perpendicular to the electrodes) and a transverse component which is negative in sign perpendicular to the filed direction. The electrostriction process is a constant volume process so the over all volume of the actuator is nearly constant even during electrical activation. The platinum electrodes attempt to impede the transverse strain or shrinkage of the actuator. Hence the elastic modulus of platinum is high (23.0×106 PSI) compared to PMN (17.5×106 PSI), the result is a potentially high shear stress component at the PNM/Pt electrode interface. In fact the shear stress is directly proportional to the magnitude of the strain and hence the applied electric field. The PMN/Pt interface is a bond which behaves much like an adhesive bond (strong in compression, moderately strong in tension, and very weak in shear). It is not surprising that nearly all electrically induced failure occurs at this interface, not in the ceramic.
Electrical connections are problemistic in the longitudinal multilayer configuration. A multilayer configuration, using the longitudinal electrostrain component, is required to provide sufficient stroke since large strain materials necessary for a single layer device exhibit significant hysteresis and limited fatigue life. All of the exposed electrodes must be connected either to ground or the field addressing source in an alternating fashion. Care must be taken to provide a ceramic insulation layer between adjacent electrodes of opposite polarity to prevent shorting the actuator when connecting the alternating electrode layers. In addition for interactuator spacing below 2.5 mm, it becomes impractical to individually route the electrical contact (both at electrodes and to the electronic driver). As the packing density increases, corresponding to better than 25 channels per cm2 (2.0 mm spacing) the manufacturing tolerance becomes very critical. In fact the ceramic shrinkage becomes a greater variable than the actual machining. Electrical interconnection becomes impractical for individual connection. In short with the current longitudinal actuator arrangement, structural stresses, electrical interconnections, and manufacturing tolerances limit the practical implementation of the multilayer actuator technology to >2.5 mm interactuator spacing. Still in order to achieve the necessary displacement, a multilayer configuration is required which leads to the aforementioned structural stress and electrical interconnect problems.